How can I intercept XMLHttpRequests from a Greasemonkey script?

Question

I would like to capture the contents of AJAX requests using Greasemonkey.

Does anybody know how to do this?

Answer 1

Not sure if you can do it with greasemonkey, but if you create an extension then you can use the observer service and the http-on-examine-response observer.

Answer 2

I have no experience of Greasemonkey (need to sort that out), but I use Fiddler when debugging this sort of thing.

Answer 3

How about modifying the XMLHttpRequest.prototype.open or send methods with replacements which set up their own callbacks and call the original methods? The callback can do its thing and then call the callback the original code specified.

In other words:

XMLHttpRequest.prototype.realOpen = XMLHttpRequest.prototype.open;

var myOpen = function(method, url, async, user, password) {
    //do whatever mucking around you want here, e.g.
    //changing the onload callback to your own version


    //call original
    this.realOpen (method, url, async, user, password);
}  


//ensure all XMLHttpRequests use our custom open method
XMLHttpRequest.prototype.open = myOpen ;

Answer 4

You can replace the unsafeWindow.XMLHttpRequest object in the document with a wrapper. A little code (not tested):

var oldFunction = unsafeWindow.XMLHttpRequest;
unsafeWindow.XMLHttpRequest = function() {
  alert("Hijacked! XHR was constructed.");
  var xhr = oldFunction();
  return {
    open: function(method, url, async, user, password) {
      alert("Hijacked! xhr.open().");
      return xhr.open(method, url, async, user, password);
    }
    // TODO: include other xhr methods and properties
  };
};

But this has one little problem: Greasemonkey scripts execute after a page loads, so the page can use or store the original XMLHttpRequest object during it's load sequence, so requests made before your script executes, or with the real XMLHttpRequest object wouldn't be tracked by your script. No way that I can see to work around this limitation.

Answer 5

Here's a little snippet that worked in my tests:

(function(open) {

    XMLHttpRequest.prototype.open = function(method, url, async, user, pass) {

        this.addEventListener("readystatechange", function() {
            console.log(this.readyState);
        }, false);

        open.call(this, method, url, async, user, pass);
    };

})(XMLHttpRequest.prototype.open);

I've used addEventListener() as Firefox throws an exception if you want to wrap the onreadystatechange property. This way you can have the cake an eat it too.